1. Field of the Invention
This invention relates to a static type circuit breaker which cuts off a fault current flowing in a current path, and in particular it relates to a static type circuit breaker in which a transistor is used as a switch element.
2. Description of the Prior Art
When short circuit trouble or the like occurs in a load circuit in the current path by which electrical power is supplied to the load, a fault current which is several times to several dozen times greater than the rated current flows in the load circuit, and there is a danger that this may cause electrical and thermal damage.
Circuit breakers which detect and cut off this fault current and provide protection for the current path have hitherto been provided with mechanical circuit breaker units in most cases.
Circuit breakers having mechanical circuit breaker units are provided with at least one pair of contact members, adapted to be separated from one another, for each phase, and to cut off the fault current by the mechanical opening and closing of these contact members. For this reason their life is short, and faults are frequent, because of the wear and deterioration of the mechanically movable parts.
Also, when the circuit is broken an arc occurs between the contact members and therefore the breaking time is long and the breaking capacity is small, and for extinguishing this arc, arc extinguishing apparatus has to be added to this circuit breaker unit, and the unit becomes bulky.
On the other hand, static type circuit breakers in which thyristors are used as circuit breaker units have been introduced, but the greatest defect of these is that, in order to cut off the fault current at high speed, commutation circuits are needed for producing a current directed oppositely to the forward direction flow to the thyristors and making their current less than the holding current. The commutation circuits call for not less than 30% of the cost and space of the whole apparatus, and they are therefore disadvantagious as regards both economy and dimensions. Also, in cases in which repeated circuit breaking is carried out, time is required for charging the condensers of the commutation circuits, and therefore there is the disadvantage that high speed repeated circuit breaking cannot be carried out.